Writing instrument



Feb. 5, 1963 c. N. JoHNsoN WRITING INSTRUMENT 2 Sheets-Sheet 1 Filed June 17, 1959 Fgb. 5, 1963 Filed June 17, 1959 E Ifo-NIFL 3y C. N. JOHNSON WRITING INSTRUMENT 2 Sheets-Sheet 2 FlG 6 www United States Patent O 3,076,438 WRITING INSTRUMENT This invention relates to ya .writing instrument .wherein the writing unit is projected forward by the force of inertia, either A,with or without the aid of gravity, a limited predetermined .dist-ance beyond the forward end or tip of said writing instrument when the projecting mechanism is actuated. In particular, this .invention relates .to a mechanical pencil wherein the writing unit or lead is projected forward by inertia va limited predetermined distance beyond .the ,tip of the pencil, `which 4distance can be adjusted ,as desired.

This application is an improvement on 4the writing instrument shown in my Patent 1No. 2,854,950, dated Qctober 7, 1958, -and has subject matter vin common with my .co-,pending application Ser. No. 751,412, lil'ed July 2,8, 1958.

The use .of extra thin lead in mechanical pencils has become increasingly popular. This lead is approximately 0.036 inch ,in diameter and is frequently broken due -to the over extension thereof from the forwardtip of a` pencil. It is estimated that over fifty percentV of such lead is wasted since it is broken off so -frequently in use. Present mechanical pencils, ,other than `shown in my Patent NO- 2,85.4,95.0, `feed the :lead from the forward end ofthe pencil in many different ways, .such as by spiral means -actuatedgby rotation, by buttonI means which when pushed Positively moves the lead forwardly step by step, or by clutch means actuated Iby rotation or by a spring lclutch as normally found in mechanical drafting pencils. With all of said present feed arrangements, ,the amount of lead which 4extends beyond the forward end of the .pencil `is not pQsitively Ameasured by means projecting externally from the pencil, ,but .is .visually .estimated `or moved by means `Wit-hill the Pencil itself. Either way, .there is no assurance that .the desired amount of lead .will extend forwardly -from the rend ,of the pencil.

In the instrument shown fin my Patent No. 2,854,950,

the lead moves by the force of gravity to and from its operative position so that the 'instrument must ,be properly positioned in order to ,effect a desired movement of the lead.

,Qne ,o .f the principal `c bjec,ts of the `present invention is to provide in a pencil of the ,type wherein the amount of projecting lead is measured b y an ,external stop, means for causing the A lead to be propelled ,by inertiaagainst said stop .so that the movement ofthe lead is not by reason of gravity alone. Another-,object is .to provide in a device of this type an arrangement which will propel .the lead and .will also produce an audible o r clicking sound which indicates to the -user that the lead has 1been prorelied Another object Vis to provide new means for adjusting the .distance that .the lead stop `will ,extend beyond tghe tip of the pencil.'

nother 'object is rv t0 Provide )a .new clutch or collet which has ,limited iopening to ,prevent .the `lead from falling out of the pencil due to misalignmentof the stopand uitgetest .thelead .from setting .Gasset ,in 'the laws of .the co et.

' nother object is to provide novel means for gripping theuservice lead over the entire contacting surface of the jaws 4beyond the forward vend of the clutch .closing tube to assureia positive grip on ,the service lead, and to provide means for preventing thelead from backing up into the instiimeht during asmel' .Wrifne Anothei' objectief the present invention is to positively ICC measure .by externally projecting means the amount of lead which extends extern-ally `downwardly from the for- Ward end of a mechanical pencil, and to maintain this amount regardless of repeated `actuation of the lead forwarding mechanism.

Another object .of the invention is to provide retractable .stop means .which is projected vbeyond the writing end of a writing instrument for preventing the writing unit from moving beyond .a predetermined .distance from the tip of 4the instrument regardless of how many `times `the lead forwarding mechanism may be actuated. Another object is to provide a writing instrument means normally positioned .within said instrument vwhich can ,be projected externally thereof to limit the distance which the writing unit is allowed to project from the tip ofthe writing instrument and to limit this in an `adjustable manner. Anotherobject is ,to provide actuating Ameans which when op,- erated causes the writing unit to move, either with or without the aid .of gravity, to its predetermined writing position in a guided manner so that it will contact the lead stop.

Another object is lto provide a `simple and ,economical commercial construction having a .self-contained writing unit assembly and a lsimple -magazine assembly which can be easily assembled into the barrel.

These and lother objects and advantages will become apparent hereinafter.

`The invention `also consists `in the parts and in the arrangement and ,combination of A.parts hereinafter described and claimed. In the accompanying ,drawings which form part of this ,specification and wherein like numerals and letters refer to like parts wherever they occur:

FIG. 1 is an enlarged longitudinal cross-sectional view of a writing instrument embodying the present invention showing the lead in its `writing position Vafter it Yhas `been worn to `some extent, the dotted lines showing the extended position of the lead stop and the extended posi-- tion `of the sleeve within the cap,

FIG. 2 is an end view vof the .writing tip of the instrument shown in FIG. -l,

HFIG. 3 is an enlarged longitudinal cross-.sectional .view of a modified form ,of the invention,

FIG. 4 is a transverse cross-sectional view taken along the Vline 4-4 of FIG. 3,

FIG. 5 is an enlarged longitudinal cross-.sectionalview of a further modified form of the invention, and

FIG. 6 lis an enlarged fragmentary view of the `inner faces .Of the clutch jaws xwith projections or `teeth therein.

Referring now .to FIGS. 1 and 2 of the `drawings in detail, iit will be seen that the .embodiment of vrthe .invention which has been illustrated comprises a writing .in-

strument 1 having a casing 2 .with a projecting mechanism /3 and `clutch 4 therein. The casing 2 comprises a ,barrel 5 and push means ,or .a .cap 6 thereon with a unitary writing unit assembly 7 and a magazine 8 therein.

The cylindrical Ibarrel 5 has internal threads 9 at its forward end for receiving the writing unit assembly 7. The 2upper portion of the barrel 5 is provided with ra plug 1,0 which slidably receives the magazine 8, Said plug 1:0 being slightly larger in diameter than the barrel 5 to ,p rovide'a ,bearing surface 11 for the inside of the push cap 6, `which has a pocket clip 12 thereon. The barrel 5 is `basically tubular and preferably cylindrical for vease in manufacturing and printing names or advertisements thereon.

The writing unit assembly 7 comprises aclutch closing tube 13, va ,clutch 4.with a forward tip 14, a lead selector 15 and various other parts. 'Ihe clutch closing tube 13 has .threads 16 thereon, a tapered outer ysuiface Y17, a'rear abutment 18, Aanda tforward portion 19. The

clutc'h closing tube has Aan opening therethrough and a rear support 21 and a forward inner surface 22. The clutch 4 slidably sets within the clutch tube 13 and is provided with a plurality of jaws 23 having inwardly projecting teeth 24 positioned rearwardly of the forward tip 14 for gripping the writing unit or lead 25 so that it does not back up into the instrument during writing. The jaws 23 preferably contact the lead 25 over their entire distance forwardly of the teeth 24. The outer inclined surfaces 26v of the jaws 23 slide with respect to the surface 22. One of said jaws 23 is provided with a slot 27 having a bottom 28. The rear portion of said clutch 4 is provided with internal threads 29 for receiving the forward end of the lead selector 15, which is provided with an enlarged head or rear portion 36 with an opening 31 therethrough and a surrounding concave or conical surface 32. The opening 31 is slightly larger, about 0.010 inch, than the lead to -be used in the pencil.

v A clutch closing spring 33 is mounted between the rear surface 13 of the clutch closing tube 13 and a rotatable clutch adjusting nut 34 for normally biasing said clutch 4 and nut 34 upwardly. A sleeve bearing 35 having a shoulder 36 thereon is slidably mounted on said lead selector 15 which -is circumscribed by a lead stop spring 37 positioned between upper surface of said clutch adjusting nut 34 and the lower surface of said shoulder 36 for normally biasing said members apart. A thin walled lead guide tube 33 on the lower portion of the lead selector 15 extends within the clutch 44 for preventing lead from adhering to the teeth 24 or other surfaces of the jaws 23. If desired, the tube 38 may be made `separate from the lead selector 15 as explained in my co-pending application Ser. No. 751,412.

A lead stop 39 is provided in the longitudinal slot 27 in the clutch 4 and extends rearwardly and terminates in a bent portion 4t) which abuts against the sleeve bearing'35. The lead stop 39 has a tip 41 which is bent angularly inwardly toward the geometric axis 42 of the writing instrument and has a sliding edge 43 thereon which slides on the bottom 28 of the slot 27 in the clutch 4. The angle between the tip 41 and the geometric axis 42 is preferably 45 or less. The lead stop 39 is made from thin resilient metal and the tip is adapted to spring lautomatically into alignment with the lead 25 to prevent the same from falling out of the opening 20 when the clutch 4 is opened and to position the amount to which it extends.

The magazine assembly 8 comprises an upper magazine 44 and an adjustable lower magazine 45 with an upper edge 46 and a bore 47 of uniform diameter. The lower magazine 45 also has a forward edge 48 in contacting relation with the sleeve type slide bearing 35, and internal threads 49 which receive external threads 56 on the upper magazine 44. The upper magazine 44 is provided with an eraser receiving chamber 51 in the upper end thereof for receiving an eraser 52. A tube 53 in the cap 6 has vertical slots therein and frictionally ts over the upper end of the magazine S and is provided with an inner bearing surface 54 to properly position the cap 6 with respect to the magazine 8 and the barrel 5. A shoulder 55 in the tube S3 is adapted to engage the upper edge of the upper magazine 44 to transmit the downward movement of the cap 6 to the magazine 8. The tube 53 has a sleeve 56 at its lower end. The sleeve 56 is pressed into the cap 6 adjacent to the bottom of the tube 53 and has its lower edge 57 positioned at a predetermined distance from the plug when the cap 6 is in its normal inoperative position. This distance, and its amount, are very important, as are the other distances hereinafter described.

It is believed that the operation of the writing instrument 1 is clear from the foregoing description, especially in view of my copending application Ser. No. 751,412. Referring to FIG. l, if it is desired to project a piece of lead 25' or change the adjustment of the worn projected lead, the cap 6 is pushed downwardly with respect to the barrel 5. With the present arrangement, the operation of the mechanism causes the lead 25 to be propelled by the force of inetria instead of being allowed to fall downwardly against the bent portion 40 of lead stop 39 solely by gravity when the pencil is held in an upright position. Even when the pencil is held in a horizontal position, the present arrangement allows the lead 25 to be propelled or shot forward after the collet jaws 23 are opened. Of cocrse, the same action occurs in all positions between the horizontal and vertical point down positions. This is accomplished by the force of inertia in the following manner.

When the cap is moved downwardly or forwardly a distance A, the lower edge 5S of the upper magazine 44 moves forward a distance B, the lower edge of the cap 6 moves forward a distance C, and the lead stop 39 moves forward a distance D. This leaves a space E if the lead has been shortened by use and there is still a space F to be traveled by the cap 6. Up to this time in the operation of the lead forwarding mechanism, the operators thumb has been opposed only by the reaction force of the relatively light lead stop spring 37. Considerably more pressure is required to close the space F, the amount of which is very critical, because the space B has already been closed by reason of the lower edge 58 of the inner or upper magazine 44 contacting the outer edge of the surface 32 on the lead selector 15, so that continued downward movement must also be against the reaction force of the stronger collet closing spring 33 as well as the lead stop spring 37.

At this time there may be a momentary pause in the operation of the pencil while enough extra force is added to the end of the cap 6 to break the clutch 4 loose from the clutch closing tube 13. This added force causes the small space F to clo-se quickly, almost instantly, and then causes the mechanism to come to a sudden stop when the bottom 57 of the sleeve 56 strikes the plug 10 instantly thereafter. As the space F is closed, the clutch 4 opens to allow the lead 25 to move freely in the enlarged axial opening 20. The whole mechanism then comes to a sudden stop with the clutch 4 still open, except for the service lead 25 which at this time is free to continue to move forward by reason of inertia to the bent portion 40 of the stop 39, since it is not restrained in any way. Of course, in order for the lead to continue to move, it must have been positioned within the lead selector 15 and guide tube 38 thereon so that as the clutch 4 moves forwardly and opens, the lead is propelled out of the end of the pencil and against the bent portion 40 of the lead stop 39.

This action is similar to the operator of an automobile suddenly putting on the brakes when the car is in motion. The car is then restrained but the occupants or any objects in the car, which are not fastened to the car, will lunge forward. The sleeve 56 causes this sudden stop when it strikes the end of the screw plug 10 in the barrel 5.

Thus, the action which causes the propulsion of the lead is caused by a combination of events in which the lead stop is first properly positioned. Then force must be built up until suddenly there is a suflicient amount at which time the cap 6 and all parts surge forward with respect to the casing to gain momentum and open the clutch and then are instantly stopped, thereby resulting in an audible click and propelling the lead forwardly.

When the pressure on the cap 6 is released, it rst causes the clutch jaws 23 to move upwardly and inwardly by reason of the clutch spring 33 thereby closing the opening so that the teeth 24 contact the properly extended lead 25 adjacent to the cam slope 26 of the jaws 23 of the clutch 4 of the instrument. Thus, the lead is fixed with respect to the clutch 4. Continued release of the pressure on the cap 6 causes the spring 37 to force the projecting mechanism and the cap 6 upwardly and move the lead ,stop 39 upwardly to its .retracted position as shown inthe full lines ,in FIG. f1.

Referring to FIG. 1, :it is seen `that the radial pressure on the clutch jaws 23 can be varied -by rotating the c lutch adjusting nut 34 to increase or decrease the pressure on the clutch spring 33. Moving the nut 34 toward the clutchclosing tube -13 compresses the spring 33 and moving the nut 34 upwardly releases the spring 33.

To adjust or vary the distance which the lead stop 39 extends beyond the tip 14, the upper magazine 44 is rotated .relative to the lower magazine 45 thereby moving the lower magazine 45 upwardly or downwardly to change the position of ysleeve bearing 35 with respect to .the writing tip. This changes the distance B `between the forward edge 58 of the upper magazine 44 and the top of the lead selector 15, since thebearing 35 is initially positioned vout of contact with the enlarged `head 30 of the lead Aselector 15 to allow adjustment in either direction. If the .upper magazine 44 is screwed into the lower magazine 45 so .that the vdistance B is decreased, the lead stop 39 cannot be extended as far .from .the writing tip 14 as when the distance B is greater.

FIGS. 3 and 4 show a modified form ofthe device shown in FIGS. 1 and 2. In this .modified form, the mechanism from' the -lead selector 15 tothe forward `tip 1 4 is substantially the same. The difference`-resides in a different magazine construction 8a made from two pieces of .thin wall tubing 44a and 45a with rolled threads thereon. A uted tube 53a -within theV cap 6 vhas a'bottorn 57a positioned to cause the sudden stop. The `tube 53a also acts as a slight friction means to hold the cap 6 onto the magazine 8a.

The .magazine M8a comprises van upper magazine 44a and an adjustable lower magazine 45a having an upper edge 46a whichY abuts against the Abottorn'iof the plug 10. The lower magazine 45a has a bore ,47 of uniform diameter which slides over the circumferential edge of the lead selector l1 5. 'The Vforward edge 48a of the lower magazine 45a is in contacting relation with the sleeve bearing 35. The upper portion of the upper magazine 44a has an eraser chamber 51a therein formed by a rolled groove 59 for holding the eraser 52.

This is a less costly construction but operates substantially the same as the device shown in FIG. 1. In operation, the cap 6 is pushed downwardly against the slight reaction force of the spring 37 to close the space B. Thereafter a surge of force must be applied to move the bottom edge 58 of the upper magazine 44a and the lead selector 15 downwardly against the additional reaction of the stronger clutch closing spring 33 to open the clutch 4. The extra force which is applied is so great and is stopped so suddenly when the bottom 57a of the tube 53a within the cap 6 Strikes the upper edge of the plug 10, that the lead 25 continues its forward travel all the way tothe lead stop 39.

Obviously the space B can be increased or decreased by threading the upper magazine 44a into or out of the lower magazine 45a. This changes the space E causing the service lead 25 to extend different distances from the writing tip of the pencil.

A further modification is shown in FIG. 5 wherein the parts are substantially the same as in the two previous forms except as follows. The lead selector 15 is provided with external threads for receiving a threaded sleeve 60 which can be rotated relative thereto. The magazine 8b has an upper portion 44b and a lower portion 45b which has an upper edge 46b adapted to contact the lower edge of the plug 10 and a bottom edge 48b adapted to contact the sleeve bearing 35. The tube 531: within the cap 6 is like that shown in FIG. 3. Thus the distance B can be varied in a different manner by rotating the threaded sleeve 60 with respect to the lead selector 15 to either increase or decrease the distance B.

Another difference is found in the clutch 4 which in this instance has a shortened inclined surface 2Gb with a straight por-tion 61 inwardly therefrom. The straight back end 61 of the clutch 4 is parallel to the axis 42 and eliminatesfthe possibility of the jaws 23a from opening too .much so as -toicaus'e the lead 25 to slip `past the bent portion 40 of the lead stop 39 and fall completely out of the pencil. It also makes the clutch 4'1ess critical to manufacture. 'This feature also prevents short pieces of thin lead getting caught between the jaws 23d jof ythe collect .as might otherwise happen. The surfaces l61 are parallel tothe geometric axis 42 so that the jaws do not open .beyond a .predetermined amount Aregardless ofthe extent of thewlongitudinalmovement.

In operation of `the deviceiin FIG. ,5, a downward force on the cap 6 causes the lower magazine 45`b to move Athe .sleeve bearing 35 downwardly against the slight reaction .of the lead stop spring 37 untilthe shoulder`62 formed in .the lower magazine 45b abuts against the upper edge of the threaded sleeve 60. A surge of Vgreater pressure must .then be applied to cause greater downward movement Vof the entire mechanism against vthe stronger force of the clutch spring 33 to open the jaws 23a a limited predetermined .amount due to the straight `outer edges 61 thereon thereby propelling or carrying Athe lead 25. with the parts until the .bottom edge 575 of the tube 53b strikes the top edge .of .thelplug '10, at which time the parts abruptly .stop 4their forward movement. However, the lead 25 therein .continues on its vway forward under inertia until it is .stopped .by the bent .portion 4t) on the lead nstop 39. 'i i In each ofthe devices shown in FIGS. 1, 3 and 5, the parts are .arranged .so .that the forward movement of the parts is abruptly stopped with .the clutch jaws 23 being in their open position .so .that the 'lead `25 can continue its forward movement by reason of inertia. At this` time, there occurs a clicking sound caused by the lower edge 57 striking the top end of the plug 10. This is desirable since it informs .theuser that the device has been actuated and the lead has been moved .toits operative position, Provided the ,pencil has been held with'the point 14 in any position from the point down position to a horizontal position.

Since it is desirable to keep the width of the lead stop 39 at a minimum, the parts must be accurately made so that the lead will not be misaligned or miss the lead s-top 39. This is accomplished by having the straight portions at the rea-r -of the clutch 4.

FIG. 6 shows an enlarged view of the inwardly projecting teeth 24 in the jaws 23 of the `clu-tch 4. 'I'he shape of the teeth is important and is designed to prevent the lead 25 from backing up into the pencil 1 by gripping it tightly. Each tooth 24 has a slanted forward portion 63 which extends rearwardly and has a straight back portion 64. Each tooth 24 extends inwardly into the opening 20 beyond the inner surface of the jaws 23 and is formed by raising a projecting portion 65 toward the axis 42 by means of radial pressure from ,a steel tile or other suitable instrument, thereby leaving a notch 66 in the jaw 23. The teeth 24 should not have a portion which bites forwardly into the lead since such a shape might tend to cut or shave the lead 25 rather than to prevent rearward movement thereof. The teeth 24 should not act as cutting edges to either shear or scrape the lead when pressure is applied during writing. The teeth 24 preferably are positioned opposite the forward portion 19 of the tube 13 and bite into the lead sufficiently so that the forwardly extending portions of the jaws 23 contact the lead over its entire lenlgth from .the teeth 24 to the forward tip 14 of the clutch 4.

It is understood that the forward end as used herein and in the trade refers to the writing end shown at the bottom of FIGS. l, 3 and 5, and -that the rear end refers to the opposite end or eraser end of the instrument, shown at the top of said gures. While lead is referred to as the writing medium herein, other writing units such as thin ball point units can be used in place of lead if desired to position the ball predetermined distance from the forward end of the instrument.

This invention is intended to cover all changes and modiicatious of the example of the invention herein chosen for purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention.

What I claim is:

1. A writing instrument having a writing assembly comprising a lead selector and a clutch connected thereto, said assembly having an opening extending therethrough for receiving lead, said clutch having jaws thereon with outer surfaces with an inclined portion and a straight portion thereon, a clutch closing tube circumscribing said clutch jaws at said surfaces, a member connected to the upper portion of said clutch, a clutch closing spring positioned between said member and said clutch closing tube for biasing the latter against said inclined surfaces, said lead selector having a head in the upper portion thereof, a lead stop extending longitudinally over a major portion of said writing assembly, said lead stop having a bent upper end portion contacting said head and a lower end portion positioned in a slot in one of said jaws, a lead stop spring positioned between said member and the bent upper end portion for normally biasing the lead stop upwardly, said lead stop spring being relatively easy to compress but requiring a major amount of movement of the push means, said clut-ch spring resisting compression until sufficient additional force is applied, stop means positioned to stop said push means within a short distance after sufficient force has been `applied to compress said clutch spring, lthereby causing an -audible sound and propelling the lead from the instrument against said external stop by reason of the inertia of the lead created therein by said movable means, said push means including two tubular pieces which can be rotated with respect to each other to adjust the distance which the lead stop projects beyond the tip of the instrument when the push means has been actuated, said clutch jaws straight portion limiting the amount of opening therebetween regardless of the amount of movement of said push means and teeth on their inner portion to bite into the lead to prevent said lead from backing up into said instrument during writing, said teeth having forward portions slanting rearwardly which extend inwardly from the inner faces of said clutch.

2. A writing instrument having a casing with an opening in the forward end thereof, a writing assembly projecting -through said opening, said instrument having a barrel and a cap, longitudinally extending push means normally positioned within said writing assembly, said writing assembly having clutch jaws therein, a relatively light lead stop spring and a relatively heavy clutch spring in said barrel which must both be compressed to open said clutch jaws, said springs positioned in contracting relation on opposite sides of a member in said writing assembly, said lead stop spring being relatively easy to compress but requiring a major amount of movement of the -push means, said clutch spring resisting compression until sufficient additional force is applied, stop means positioned in the upper portion of the barrel to stop said push means within a short distance after suicient force has been applied to compress said clutch spring, thereby causing an audible sound land propelling the lead from the instrument against said external s-top by reason of the inertia of the lead created therein by said movable means, said push means including two tubular pieces which can be rotated with respect to each other to adjust the distance whic-h `the lead stop projects beyond the tip of the instrument when the push means has been actuated, said clutch jaws having a straight portion on their outer cam surfaces to limit the amount of opening therebetween regardless ofthe amount of movement of said push means.

References Cited in the tile of this patent UNITED STATES PATENTS 

2. A WRITING INSTRUMENT HAVING A CASING WITH AN OPENING IN THE FORWARD END THEREOF, A WRITING ASSEMBLY PROJECTING THROUGH SAID OPENING, SAID INSTRUMENT HAVING A BARREL AND A CAP, LONGITUDINALLY EXTENDING PUSH MEANS NORMALLY POSITIONED WITHIN SAID WRITING ASSEMBLY, SAID WRITING ASSEMBLY HAVING CLUTCH JAWS THEREIN, A RELATIVELY LIGHT LEAD STOP SPRING AND A RELATIVELY HEAVY CLUTCH SPRING IN SAID BARREL WHICH MUST BOTH BE COMPRESSED TO OPEN SAID CLUTCH JAWS, SAID SPRINGS POSITIONED IN CONTRACTING RELATION ON OPPOSITE SIDES OF A MEMBER IN SAID WRITING ASSEMBLY, SAID LEAD STOP SPRING BEING RELATIVELY EASY TO COMPRESS BUT REQUIRING A MAJOR AMOUNT OF MOVEMENT OF THE PUSH MEANS, SAID CLUTCH SPRING RESISTING COMPRESSION UNTIL SUFFICIENT ADDITIONAL FORCE IS APPLIED, STOP MEANS POSITIONED IN THE UPPER PORTION OF THE BARREL TO STOP SAID PUSH MEANS WITHIN A SHORT DISTANCE AFTER SUFFICIENT FORCE HAS BEEN APPLIED TO COMPRESS SAID CLUTCH SPRING, THEREBY CAUSING AN AUDIBLE SOUND AND PROPELLING THE LEAD FROM THE INSTRUMENT AGAINST SAID EXTERNAL STOP BY REASON OF THE INERTIA OF THE LEAD CREATED THEREIN BY SAID MOVABLE MEANS, SAID PUSH MEANS INCLUDING TWO TUBULAR PIECES WHICH CAN BE ROTATED WITH RESPECT TO EACH OTHER TO ADJUST THE DISTANCE WHICH THE LEAD STOP PROJECTS BEYOND THE TIP OF THE INSTRUMENT WHEN THE PUSH MEANS HAS BEEN ACTUATED, SAID CLUTCH JAWS HAVING A STRAIGHT PORTION ON THEIR OUTER CAM SURFACES TO LIMIT THE AMOUNT OF OPENING THEREBETWEEN REGARDLESS OF THE AMOUNT OF MOVEMENT OF SAID PUSH MEANS. 